This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Inductive elements where the inductance and/or resistance may not be known or may vary include such elements as DC motors, solenoids, and voice coils. These elements are inherently inductive which means that changes in an applied coil voltage do not instantaneously result in a change in the coil current. The ability to change the coil current quickly is important in order to quickly control the voice coil mechanical elements.
In a voice coil-actuated pneumatic valve, it is important that current be accurately controlled in order to accurately control the valve. Stability and transient response of the valve current are closely related to the electrical load (i.e., inductance and resistance of the voice coil) as well as to the voltage of the power supply. Controller gains that lead to acceptable performance for one voice coil and supply voltage, therefore, may yield highly unsatisfactory performance for another.
In general, classical solutions to compensate for an uncertain plant or load, in a control system would lead to increased system complexity. For a minimal closed-loop current controller, only current must be sensed and used as feedback. Another, simple way of compensating for power supply voltage and load is to sense power supply voltage directly. Power supply voltage feedback, in conjunction with load current feedback, can be used in the controller for compensation. The disadvantage to this method is that using it would require an additional sensing circuit (which would consume microcontroller peripherals, printed circuit board space, and add complexity to the circuit design), as well as a load-identification algorithm.
An additional classical solution to this problem would be to make the current controller tunable (or selectable) by the user, thereby allowing for a wide range of loads and power supplies. Unfortunately, this adds another step for the user before the controller can be used. Also, since this method is not automatic, such a controller must be re-tuned when used in a system with a different set of parameters, for instance, if the power supply voltage or voice-coil parameters change.
Due in part to manufacturing variances in voice coil motor construction, broad offerings of products with voice coil motors of different characteristics, and the need to allow the customer the ability to operate voice coil products with a wide range of supply voltages, there is a need for a voice coil controller which can accommodate a wide range of loads and power supplies without re-tuning when used in a new system. Such a controller should automatically operate to a given set of specifications for many different power supply voltages and loads.